1. Field Of the Invention
The present invention relates to a steam box for cross-machine profiled moisturizing and heating of a web of paper. The steam box includes a steam distributing system for distributing the steam onto the web being calendered. At least one valve is connected to the steam distributing system for controlling the flow of steam onto the web. In addition, elements are provided for feeding hot steam to the valve.
2. Description of the Related Art
Paper web gloss after calendering can be improved by heating and moisturizing of the web with steam prior to calendering or between calendering. Moisturizing with steam can be used in conjunction with all calendering methods. The steam is directed onto the web via a steam box. Due to the variations in the properties of the web to be calendered, the steam box is conventionally divided in the cross-machine direction of the web into independently controllable compartments. The compartmentally controllable steam feed can be employed for equalizing the moisture profile of the web and reducing the effect of thickness and texture variations on the smoothness and gloss of the calendered web. The controllable steam feed can also be utilized for removal of one-sidedness of the web texture by moisturizing the coarser side of the web with more steam than the opposite smoother side. The chief goal of steam-treatment is, however, to attain improvement of paper gloss after calendering and homogeneous gloss over the entire web width.
The steam feed necessary for achieving a sufficient gloss on coated paper grades is relatively small, typically 2-10 kg/m/h. For uncoated paper grades the steam feed is greater, i.e., 20-100 kg/m/h. Feeding small amounts of steam onto a fast moving web causes several. problems. Frequently, the steam feed onto the web must be set greater than the absorbing capability of the web. Steam can then leak to the vicinity of the steam feed point, where it can rapidly condense on any cooler surfaces. Such steam leakage is inevitable in all conventional steam boxes. Particularly disadvantageous is the condensation of steam on cooler areas of a web to be calendered and of the calender rolls. This results in excessive moisturization of the web, and consequently, degradation of the web surface quality. Excessive moisture on the web surface causes coat blistering and adherence on the calender rolls. Condensed moisture easily forms droplets, which when reaching the web surface ruin the coat.
In profile-correcting steam applications, the steam feed is compartmentally switched on and off according to the steam requirements. If steam feed is switched off from any compartment or the entire steam box, the great amount of air carried along with the fast moving web quickly cools compartments of the steam box. Consequently, water can easily condense in the box and be directed onto the web when the steam feed is again switched on. Such dripping causes defects on the coated paper web. A corresponding situation occurs when steam is switched on at cold start. Prevention of condensate formation and its subsequent splattering on the web to be coated has been attempted by means of heating the box with steam or electricity and using different types of water collecting channels and water drainage unions mounted to the channels.
U.S. Pat. No. 4,786,569 discloses an apparatus in which excess moisturizing steam is prevented from escaping from the steam box to the surroundings by a suction apparatus. However, while the use of a suction apparatus can prevent the spreading of the steam into the surroundings, its lacking capability of the prevention of condensation in intermittent use causing subsequent formation of droplets makes such an apparatus poorly suited to intermittent use and cross-machine control of the moisture profile of the web.
European Patent Application 0,380,413 describes a steam-feed apparatus in which condensate is prevented from dropping onto the web. This apparatus has an adjustable valve for each steam-blowing compartment, capable of providing flow control for the amount of steam directed onto the web. The steam is routed from the valve into a deflecting header in which its velocity vector is altered, so that any condensate possibly following the steam is separated and can be led away from the steam-feed apparatus. The construction of such an apparatus and its nozzles and deflecting headers in particular is complicated, and due to the structure of the deflecting header, the apparatus must be installed so that steam is directed upward from the nozzles of the deflecting header. Thus, such an apparatus must always be placed below the web, which curtails its applications and makes, e.g., simultaneous two-sided blowing of steam on the web impossible. Application of steam from above the web is frequently needed, particularly in soft-nip calendering. Moreover, prevention of droplets from dropping onto the web has been attempted by way of placing the steam-feed apparatus relatively far from the web. This arrangement, however, results in strong cooling of the steam by the air layer carried along with the web before the steam can impinge on the web, thus causing an inferior heating effect of the steam on the web. Moreover, steam can condense into water as the steam temperature falls, so that the water droplets will be directly blown onto the web, thus ruining the coat.
U.S. Pat. No. 4,945,654 discloses an apparatus in which condensation into droplets is prevented by continuously maintaining the apparatus temperature above 100.degree. C. Steam temperature is elevated sufficiently by means of heater elements placed in the steam-feed chamber of the apparatus. Keeping the apparatus at a sufficiently elevated temperature and heating the steam in the steam-feed chamber consumes large amounts of energy, thus rendering the use of such an apparatus inefficient.
Further, a steam nozzle construction is known in the art resembling the suction nozzles used for drying a paper web. In such an embodiment steam is jetted against the machine direction of the web into a gap between the web and an adjacent plane. The plane and the exit edge of the nozzle are kept warm by the jetted steam, thereby aiming to prevent condensation of the steam into droplets and subsequent landing of droplets onto the web. Such a nozzle structure achieves jetting of the steam onto the web at a close distance and, consequently, effective heat transfer. However, if any nozzle over a web section remains unused for some time, the steam chamber communicating with its nozzle slot can accumulate condensate which is easily jetted onto the web at the restart of the steam-blowing. Moreover, the exit edge of the nozzle has relatively complicated heating arrangements. The amount of blown steam is altered by changing the amount of steam entering the nozzles. Therefore, the steam flow rates in the steam chambers of the nozzles vary.